Sub-sampling Frequency Converter
Let us recall that the frequency down conversion consists in shifting the input signal with a frequency fin down to a lower frequency fout, without altering its amplitude or frequency modulation. One interesting solution consists in using a transmission gate with a very accurate tuning of the gate clock.
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| Figure 12-92. Principles of down conversion |
As an illustration, we use a 1.900 GHz sinusoidal wave (DataIn), and a 1.818 GHz sampling signal (Enable). The expected output frequency is therefore 1.900-1.818=0.082 GHz, that is 82MHz. The layout of the sample circuit is a simple transmission gate with an RC filter (Figure 12-93).
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| Figure 12-93. Layout of the transmission
gate and RC filter used for down conversion |
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| Figure 12-94. Down conversion of the
1.9GHz input sinusoidal wave to a low frequency near 82MHz |
The sampling signal Enable operates at a rate slightly slower than
the input frequency, which leads to a signal at low frequency at the output
of the transmission gate (Figure 12-94). With a simple RC filtering, the output
signal becomes a sinusoidal wave with a frequency equal to the difference
between fDataIn and the gate frequency fEnable. When simulating over
a 20ns time interval (Figure 12-94), and asking for the evaluation of the
frequency of the output subSample, we find 82MHz as expected.
